Method to increase performance of secondary data in a hierarchical modulation scheme

ABSTRACT

The present invention provides a receiver for use in a SDAR system, the receiver including a receiving unit having satellite signal detection means for detecting a first transmit signal transmitted from a first communication satellite and a second transmit signal transmitted from a second communication satellite, the first transmit signal produced when the transmitter modulates a primary data stream with a secondary data stream on a first carrier wave associated with the first communication satellite and the second transmit signal produced when the transmitter modulates the primary and secondary data streams on a second carrier wave associated with the second communication satellite; and at least one demodulator coupled to the receiving unit and configured to demodulate the at least one of the first and the second transmit signals.

This application claims the benefit of U.S. Provisional PatentApplication No. 60/525,616 filed on Nov. 26, 2003.

TECHNICAL BACKGROUND

The present invention generally relates to the transmission of digitaldata, and more particularly, to the transmission of digital data in asatellite digital audio radio (“SDAR”) system.

BACKGROUND OF THE INVENTION

In October of 1997, the Federal Communications Commission (FCC) grantedtwo national satellite radio broadcast licenses. In doing so, the FCCallocated twenty-five (25) megahertz (MHz) of the electromagneticspectrum for satellite digital broadcasting, twelve and one-half (12.5)MHz of which are owned by XM Satellite Radio, Inc. of Washington, D.C.(XM), and 12.5 MHz of which are owned by Sirius Satellite Radio, Inc. ofNew York City, N.Y. (Sirius). Both companies provide subscription-baseddigital audio that is transmitted from communication satellites, and theservices provided by these and other SDAR companies are capable of beingtransmitted to both mobile and fixed receivers on the ground.

In the XM satellite system, two (2) communication satellites are presentin a geostationary orbit—one satellite is positioned at longitude 115degrees (west) and the other at longitude eighty-five (85) degrees(east). Accordingly, the satellites are always positioned above the samespot on the earth. In the Sirius satellite system, however, three (3)communication satellites are present that all travel on the same orbitalpath, spaced approximately eight (8) hours from each other.Consequently, two (2) of the three (3) satellites are “visible” toreceivers in the United States at all times. Since both satellitesystems have difficulty providing data to mobile receivers in urbancanyons and other high population density areas with limitedline-of-sight satellite coverage, both systems utilize terrestrialrepeaters as gap fillers to receive and re-broadcast the same data thatis transmitted in the respective satellite systems.

In order to improve satellite coverage reliability and performance, SDARsystems currently use three (3) techniques that represent differentkinds of redundancy known as diversity. The techniques include spatialdiversity, time diversity and frequency diversity. Spatial diversityrefers to the use of two (2) satellites transmitting near-identical datafrom two (2) widely-spaced locations. Time diversity is implemented byintroducing a time delay between otherwise identical data, and frequencydiversity includes the transmission of data in different frequencybands. SDAR systems may utilize one (1), two (2) or all of thetechniques.

The limited allocation of twenty-five (25) megahertz (MHz) of theelectromagnetic spectrum for satellite digital broadcasting has createda need in the art for an apparatus and method for increasing the amountof data that may be transmitted from the communication satellites to thereceivers in SDAR systems.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for increasingthe amount of digital data that may be transmitted from communicationsatellites to receivers in SDAR systems. In doing so, the presentinvention provides an advantage over the prior art. While hierarchicalmodulation schemes have been previously used in other data transmissionapplications (e.g., Digital Video-Broadcasting—Terrestrial [DVB-T] andDVB-Satellite [DVB-S] systems), until now, such hierarchical modulationschemes have not been envisioned for use in SDAR systems. By introducingthe use of hierarchical modulation in SDAR systems, the presentinvention increases the amount of data that may be transmitted in SDARsystems and enables the enhanced performance of the receivers thatreceive the satellite-transmitted signals in SDAR systems.

In one form of the present invention, a receiver is provided, thereceiver including a receiving unit having satellite signal detectionmeans for detecting a first transmit signal transmitted from a firstcommunication satellite and a second transmit signal transmitted from asecond communication satellite, the first transmit signal produced whenthe transmitter modulates a primary data stream with a secondary datastream on a first carrier wave associated with the first communicationsatellite and the second transmit signal produced when the transmittermodulates the primary and secondary data streams on a second carrierwave associated with the second communication satellite; and at leastone demodulator coupled to the receiving unit and configured todemodulate the at least one of the first and the second transmitsignals.

In another form of the present invention, a method of receiving data isprovided, the method including the steps of detecting a first transmitsignal, the first transmit signal transmitted from a first communicationsatellite and comprising a first level data and a second level datamodulated on a first carrier wave associated with the firstcommunication satellite; detecting a second transmit signal, the secondtransmit signal transmitted from a second communication satellite andcomprising the first level data and the second level data modulated on asecond carrier wave associated with the second communication satellite;interpreting the first transmit signal; and providing the interpretedsignal to an output unit.

In still another form, the present invention provides a terrestrialrepeater used to re-transmit hierarchically modulated data, the repeaterincluding a receiving unit for detecting a first transmit signal and asecond transmit signal, the first transmit signal transmitted from afirst communication satellite and produced when the transmittermodulates a primary and a secondary data on a first carrier waveassociated with the first communication satellite, and the secondtransmit signal transmitted from a second communication satellite andproduced when the transmitter modulates the primary and the secondarydata on a second carrier wave associated with the second communicationsatellite; an encoder for re-encoding the primary and the secondary dataon a third carrier wave to form a third transmit signal; and are-transmitter for transmitting the third transmit signal to a receiver.

In yet another form of the present invention, a method of receivingtransmitted data, the method including the steps of receiving a primarydata stream and a secondary data stream from both a first communicationsatellite and a second communication satellite, the primary data streamhaving a first data rate and the secondary data stream having a seconddata rate; determining the validity of the first and the second datastreams; and hierarchically modulating the primary and the secondarydata streams to form a combined data stream when the secondary datastream is determined to be an enhancement to the primary data stream,wherein the combined data stream has a third data rate greater than thefirst data rate.

BRIEF DESCRIPTION OF THE DRAWING

The above-mentioned and other features and objects of this invention,and the manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescription of embodiments of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is an illustrative view of a constellation chart for64-quadrature amplitude modulation (QAM) with an embedded quadraturephase shift keying (QPSK) stream;

FIG. 2 is a diagrammatic view of a SDAR system implementing a method ofthe present invention;

FIG. 3 is a block diagram of a SDAR communication system adapted toenable a method of the present invention;

FIG. 4 is a diagrammatic view of a QPSK constellation;

FIG. 5 is a diagrammatic view of a binary phase shift keying (BPSK)constellation;

FIG. 6 is a diagrammatic view of a hierarchical 8-PSK constellation; and

FIG. 7 is a flow chart illustrating a method of the present invention asutilized in a SDAR receiver.

Corresponding reference characters indicate corresponding partsthroughout the several views. Although the drawings representembodiments of the present invention, the drawings are not necessarilyto scale and certain features may be exaggerated in order to betterillustrate and explain the present invention. The exemplifications setout herein illustrate embodiments of the invention in several forms andsuch exemplification is not to be construed as limiting the scope of theinvention in any manner.

DESCRIPTION OF INVENTION

The embodiments disclosed below are not intended to be exhaustive orlimit the invention to the precise forms disclosed in the followingdetailed description. Rather, the embodiments are chosen and describedso that others skilled in the art may utilize their teachings.

For the purposes of the present invention, certain terms shall beinterpreted in accordance with the following definitions.

“Feed forward correction” is a method of improving secondary data(defined infra) detection. By knowing the relative “I” (in-phase) and“Q” (quadrature) components of a constellation quadrant, the detectormay be enhanced to perform better by having a priori knowledge to assistdetection.

“First level data” and/or “primary data” hereinafter refers to existingdata that may be interpreted by current (i.e., “legacy”) SDAR receivers.Because the first level data can be interpreted by the legacy receivers,the first level data may also be considered to have backwardscompatibility.

“Hierarchical modulation” hereinafter describes when two separate dataor bit streams are modulated onto a single data stream. Essentially, anadditional data stream is superimposed upon, mapped on, or embeddedwithin the primary data transmission. The additional data stream mayhave a different data rate than the primary data stream. As such, theprimary data is more susceptible to noise than it would be in anon-hierarchical modulation scheme. By using a different codingalgorithm, the usable data of the additional stream may be transmittedwith a different level of error protection than the primary data stream.Broadcasters of SDAR services may use the additional and primary datastreams to target different types of receivers, as will be explainedbelow.

“Legacy receiver” hereinafter describes a current or existing SDARreceiver that is capable of interpreting first level data. Legacyreceivers typically interpret second level data as noise.

“Second generation receiver” hereinafter describes a SDAR receiver thatcontains hardware and/or software enabling the receiver to interpretsecond level data (e.g., demodulator enhancements). Second generationreceivers may also interpret first level data.

“Second level data”, “secondary data” and/or “hierarchical data”hereinafter refers to the additional data that is superimposed on thefirst level data to create a hierarchically modulated data stream.Second level data may be interpreted by SDAR receivers containing theappropriate hardware and/or software to enable such interpretation(i.e., “second generation” receivers). Second level, or secondary, datamay perform differently from first level, or primary, data.

QAM is one form of multilevel amplitude and phase modulation that isoften employed in digital data communication systems. Using atwo-dimensional symbol modulation composed of a quadrature (orthogonal)combination of two (2) pulse amplitude modulated signals, a QAM systemmodulates a source signal into an output waveform with varying amplitudeand phase. Data to be transmitted is mapped to a two-dimensional,four-quadrant signal space, or constellation. The QAM constellationemploys “I” and “Q” components to signify the in-phase and quadraturecomponents, respectively. The constellation also has a plurality ofphasor points, each of which represent a possible data transmissionlevel. Each phasor point is commonly called a “symbol,” represents bothI and Q components and defines a unique binary code. An increase in thenumber of phasor points within the QAM constellation permits a QAMsignal to carry more information.

Many existing systems utilize QPSK modulation systems. In such QPSKsystems, a synchronous data stream is modulated onto a carrier frequencybefore transmission over the satellite channel, and the carrier can havefour (4) phase states, e.g., 45 degrees, 135 degrees, 225 degrees or 315degrees. Thus, similar to QAM, QPSK employs quadrature modulation wherethe phasor points can be uniquely described using the I and Q axes. Incontrast to QAM, however, the pair of coordinate axes in QPSK can beassociated with a pair of quadrature carriers with a constant amplitude,thereby creating a four (4) level constellation, i.e., four (4) phasorpoints having a phase rotation of 90 degrees. Differential quadraturephase shift keying (D-QPSK) refers to the procedure of generating thetransmitted QPSK symbol by calculating the phase difference of thecurrent and the preceding QPSK symbol. Therefore, a non-coherentdetector can be used for D-QPSK because it does not require a referencein phase with the received carrier.

Hierarchical modulation, used in DVB-T systems as an alternative toconventional QPSK, 16-QAM and 64-QAM modulation methods, may better beexplained with reference to FIG. 1. FIG. 1 illustrates 64-QAMconstellation 100. Each permissible digital state is represented byphasors 110 in the I/Q plane. Since eight (8) by eight (8) differentstates are defined, sixty-four (64) possible values of six (6) bits maybe transmitted in 64-QAM constellation 100. FIG. 1 shows the assignmentof binary data values to the permissible states. In a 16-QAMconstellation, there are four (4) by four (4) different states and four(4) transmitted bits, in a 4-PSK constellation, there are two (2) by two(2) states and two (2) transmitted bits, and in a BPSK constellation,there is one (1) state and one (1) transmitted bit.

In systems employing hierarchical modulation schemes, the possiblestates are interpreted differently than in systems using conventionalmodulation techniques (e.g., QPSK, 16-QAM and 64-QAM). By treating thelocation of a state within its quadrant and the number of the quadrantin which the state is located as a priori information, two separate datastreams may be transmitted over a single transmission channel. While64-QAM constellation 100 is still being utilized to map the data to betransmitted, it may be interpreted as the combination of a 16-QAM and a4-PSK modulation. FIG. 1 shows how 64-QAM constellation 100, upon whichis mapped data transmitted at six (6) bits/symbol 116, may beinterpreted as including QPSK constellation 112 (which includes mappeddata transmitted at two (2) bits/symbol) combined with 16-QAMconstellation 114 (which includes mapped data transmitted at four (4)bits/symbol). The combined bit rates of QPSK and the 16-QAM data steamsis equal to the bit rate of the 64-QAM data stream.

In systems employing hierarchical modulation schemes, one (1) datastream is used as a secondary data stream while the other is used as aprimary data stream. The secondary data stream typically has a lowerdata rate than the primary stream. Again referring to FIG. 1, using thishierarchical modulation scheme, the two (2) most significant bits 118may be used to transmit the secondary data to second generationreceivers while the remaining four (4) bits 119 may be used to code theprimary data for transmission to the legacy receivers.

The present invention contemplates the use of hierarchical modulation ina SDAR system, while maintaining backward compatibility for legacyreceivers. Shown in FIG. 2 is a diagrammatic view of a SDAR system inwhich a hierarchical modulation scheme is employed. SDAR system 210includes first and second communication satellites 212, 214, whichtransmit line-of-sight signals to SDAR receivers 216, 217 located on theearth's surface. A third satellite may be included in other SDARsystems. Satellites 212, 214, as indicated above, may provide forspatial, frequency and time diversity. As shown, receiver 216 is aportable receiver such as a handheld radio or wireless device. Receiver217 is a mobile receiver for use in vehicle 215. SDAR receivers 216, 217may also be stationary receivers for use in a home, office or othernon-mobile environment.

SDAR system 210 further includes a plurality of terrestrial repeaters218, 219. Terrestrial repeaters 218, 219 receive and retransmit thesatellite signals to facilitate reliable reception in geographic areaswhere the satellite signals are obscured from the view of receivers 216,217 by obstructions such as buildings, mountains, canyons, hills,tunnels, etc. The signals transmitted by satellites 212, 214 andterrestrial repeaters 218, 219 are received by receivers 216, 217, whicheither combine or select one of the signals as receiver's 216, 217output.

FIG. 3 illustrates a block diagram of a SDAR communication system inwhich hierarchical modulation is utilized. In an exemplary embodiment ofthe present invention, SDAR communication system 300 includes SDARtransmitter 310, SDAR receiver 340 and terrestrial repeater 350. As inconventional SDAR communication systems, SDAR communication system 300will input data content 302, 304 and perform processing and frequencytranslation within transmitter 310. The digital data is transmitted overtransmission channel 330 to receiver 340 or terrestrial repeater 350.Generally, receiver 340 performs the converse operations of transmitter310 to recover data 302, 304. Repeater 350 generally re-transmits data302, 304 to receiver 340. Unlike conventional SDAR communicationsystems, however, transmitter 310, receiver 340 and repeater 350 of thepresent invention provide hardware enabling SDAR communication system300 to utilize a hierarchical modulation scheme to transmit and receivemore digital data than conventional systems.

SDAR transmitter 310 includes encoders 312, 322. The audio, video, orother form of digital content to be transmitted comprises primary inputsignal 302 and secondary input signal 304, which are typically arrangedas series of k-bit symbols. Primary input signal 302 contains primary,or first level, data and secondary input signal 304 contains secondary,or second level, data. Encoders 312, 322 encode the k bits of eachsymbol as well as blocks of the k-bit symbols. In other embodiments ofthe present invention, separate encoders may be used to encode theblocks of k-bit symbols, for example, outer and inner encoders. In anexemplary embodiment of the present invention, encoder 312 may encodeprimary data stream 302 using a block or a convolutional forward errorcorrection (FEC) algorithm, and encoder 322 may encode secondary datastream 304 using a turbo coding algorithm or a low density parity checkFEC algorithm. It is contemplated that other FEC encoding methods may beutilized to encode primary and secondary data streams 302, 204,including, for example, Hamming codes, cyclic codes and Reed-Solomon(RS) codes.

Again referring to FIG. 3, inner interleaver 316 multiplexes encodedsecondary content data stream 304 with encoded primary content datastream 302 to form a transmit data stream. This transmit data stream ispassed to mapper 317, which maps the data stream into symbols composedof I and Q signals. Mapper 317 may be implemented as a look-up tablewhere sets of bits from the transmit signal are translated into I and Qcomponents representing constellation points or symbols. FIG. 6 isrepresentative of an exemplary embodiment of the present invention, inwhich a hierarchical-modulation scheme is employed and the constellationpoints are in accordance with either a uniform or non-uniform 8-PSKconstellation 600, where each phasor is represented by a three (3) bitsymbol composed of I and Q signals.

FIG. 4 shows QPSK constellation 400 for primary data having two (2)transmitted bits/symbol. Phasors “00”, “10”, “11 ”, “01” correlate to aphase of 45 degrees, a phase of 135 degrees, a phase of 225 degrees anda phase of 315 degrees, respectively. FIG. 5 shows BPSK constellation500 for secondary data having one (1) transmitted bit/symbol. Phasors“0” and “1” correlate to a phase of zero (0) and 180 degrees,respectively. When a secondary data symbol is added onto a primary datasymbol, constellation 600 of FIG. 6 is illustrative of the resultinghierarchical modulation.

Constellation 600 may be perceived as two (2) sets of superimposedmodulations—QPSK constellation 400 transmitting two (2) bits/symbol 620combined with BPSK constellation 500 comprising one (1) bit/symbol . Thefirst modulation is the primary QPSK data, which is represented by “x”marks 620, 622, 624, 626. In order to superimpose the secondary dataonto the primary data, the primary QPSK data is phase offset by theadditional, secondary data, which is represented by any of data points601, 602, 603, 604, 605, 606, 607, 608 depending on the phase offset.Positive phase offsets include phasors 602, 604, 606 and 608, andnegative phase offsets include 601, 603, 605 and 607.

Shown in FIG. 6, phase offset 610 is the offset angle relative to theQPSK symbol. As explained above, a typical QPSK constellation contains45 degree, 135 degree, 225 degree and 315 degree points. Thehierarchical data is represented by a phase offset relative to thosefour (4) degree points, and the phase offsets with the four (4) degreepoints represent a hierarchical (8-PSK) constellation. A uniform 8-PSKconstellation is created when offset angle 610 is 22.5 degrees. Everyother offset angle 610 creates a non-uniform 8-PSK constellation. Forexample, as shown in FIG. 6, a 15 degree phase offset relative toprimary data phasors 620, 622, 624, 626 produces a phase offsetcorrelative to phasors 601 (“000”) or 602 (“001”), 603 (“101”) or 604(“100”), 605 (“110”) or 606 (“111”), and 607 (“011”) or 608 (“010”),respectively. Gray coding is a method which may be used to make the bitassignments for the hierarchical constellation. For example, referenceis made to the secondary data bit (b2). Instead of making b2=0 anegative offset and b2=1 a positive outset, the hierarchicalconstellation may be configured so as to increase the bit error rate(BER) performance (e.g., b2=1 can be made a negative offset).

The amount of the phase offset is equal to the amount of power in thesecondary signal. The amount of energy in the secondary signal may notbe equal to the amount of energy in the primary signal. As phase offset610 is increased, the energy in the secondary data signal is alsoincreased. The performance degradation to the primary data signal isminimized by the perceived coding gain improvement as phase offset 610is increased. The application of the hierarchical phase modulation ontop of an existing QPSK signal containing primary data causes phaseoffset 610 to adjust either positively or negatively relative to thehierarchical data.

In general, a secondary data bit causes either a larger Q magnitude andsmaller I magnitude or a larger I magnitude and smaller Q magnitude.With FEC techniques utilized in encoders 312, 322, the I and Q signalsare used in conjunction with each other over a block of data. Thesetechniques give the appearance that the primary data bits are spreadover time, enabling the secondary data to appear somewhat orthogonal tothe primary data bits. Indeed, it has been shown in simulations that thesecondary data's impact on the primary data is somewhat orthogonal. Forexample, for a twenty (20) degree phase offset for secondary data, theprimary data has a one (1) decibel (dB) degradation when using a rate1/3 convolutional code with a constraint length of seven (7), followedby a (255, 223) RS block code (8 bits/symbol). However, when the primarydata has no FEC coding, the impact of the twenty (20) degree phaseoffset is 4.1 dB. This data demonstrates a perceived coding improvementof 3.1 dB in the case where phase offset 610 is set to twenty (20)degrees.

Again referring to FIG. 3, the FEC coding technique implemented byencoders 312, 322 spreads the primary and secondary data over many QPSKsymbols, which essentially spreads the energy over time and the I and Qbits. To overcome the unequal signal-to-noise ratio (“Eb/No”) betweenprimary data bits and secondary data bits, the amount of phase offset610 may be increased until the performance of the primary data is equalto the performance of the secondary data. However, as phase offset 610is increased, legacy receivers may have a difficult time acquiring andtracking the desired primary data signal. By spreading the second levelbits over multiple symbols, spread spectrum coding techniques may beused to increase the amount of energy in the secondary bits. This allowsphase offset 610 to be adjusted and made more compatible with legacyreceivers. Additionally, the use of second level data spreading reducesoverall second level data throughput. Overall, several techniques may beutilized to maximize the performance of the secondary data. Thesetechniques include: increasing phase offset 610 to maximize thesecondary data energy per symbol; using multiple symbols per secondarydata bit; using more complex FEC algorithms, and using a beam steeringantenna to improve the performance of the secondary data (e.g., a highergain directional antenna for stationary reception and apointing/steering antenna for mobile reception).

Referring back to FIG. 3, after mapper 317 translates encoded andinterleaved primary and secondary data streams 302, 304, respectively,into I and Q components, the I and Q components are modulated bymodulator 318. Modulation enables both primary data stream 302 andsecondary data stream 304 to be transmitted as a single transmissionsignal via antenna 326 over single transmission channel 330. Primarydata stream 302 is modulated with secondary data stream 304 using one ofa number of modulation techniques, including BPSK, QPSK, differentialQ-PSK (D-QPSK) or pi/4 differential QPSK (pi/4 D-QPSK). According to thetechnique that modulator 318 employs, modulator 318 may be any of aQPSK, a D-QPSK or a pi/4 D-QPSK modulator. Each modulation technique isa different way of transmitting the data across channel 330. The databits are grouped into pairs, and each pair is represented by a symbol,which is then transmitted across channel 330 after the carrier ismodulated.

An increase in the capacity of the transmitted signal would not causebackwards compatibility problems with legacy receivers because thelegacy receivers may interpret the first level data. Second generationreceivers, however, are capable of interpreting both first and secondlevel data. Techniques may be employed to minimize the degradation inthe legacy receiver, including decreasing phase offset 610 to limit theamount of the second level data energy per symbol, limiting the amountof time over which the second level data is transmitted, and making thesecond level data energy appear as phase noise to the legacy receiver.

Referring back to FIG. 2, after modulator 318 modulates first datastream 302 and second level data stream 304 (FIG. 3) to create atransmission signal, transmitter 213 uplinks the transmission signal tocommunication satellites 212, 214. Satellites 212, 214, having a “bentpipe” design, receive the transmitted hierarchically modulated signal,performs frequency translation on the signal, and re-transmits, orbroadcasts, the signal to either one or more of plurality of terrestrialrepeaters 218, 219, receivers 216, 217, or both.

As shown in FIG. 3, terrestrial repeater 350 includes terrestrialreceiving antenna 352, tuner 353, demodulator 354, de-interleaver 357,modulator 358 and frequency translator and amplifier 359. Demodulator354 is capable of down-converting the hierarchically modulateddownlinked signal to a time-division multiplexed bit stream, andde-interleaver 357 re-encodes the bit-stream in an orthogonal frequencydivision multiplexing (OFDM) format for terrestrial transmission. OFDMmodulation divides the bit stream between a large number of adjacentsubcarriers, each of which is modulated with a portion of the bit streamusing one of the M-PSK, differential M-PSK (D-MPSK) or differential pi/4M-PSK (pi/4 D-MPSK) modulation techniques. Accordingly, if ahierarchically modulated signal is transmitted to one or bothterrestrial repeaters 218, 219 (FIG. 2), terrestrial repeaters 218, 219receive the signal, decode the signal, re-encode the signal using OFDMmodulation and transmit the signal to one or more receivers 216, 217.Because the signal contains both the first and second level data, theterrestrial signal maintains second level data bit spreading overmultiple symbols.

Also shown in FIG. 3, SDAR receiver 340 contains hardware (e.g., achipset) and/or software to process any received hierarchicallymodulated signals as well. Receiver 340 includes one or more antennas342 for receiving signals transmitted from either communicationsatellites 212, 214, terrestrial repeaters 218, 219, or both (FIG. 2).Receiver 340 also includes tuner 343 to translate the received signalsto baseband. Separate tuners may be used to downmix the signals receivedfrom communication satellites 212, 214 and the signals received fromterrestrial repeaters 218, 219. It is also envisioned that one tuner maybe used to downmix both the signals transmitted from communicationsatellites 212, 214 and the signals transmitted from repeaters 218, 219.

Once the received signal is translated to baseband, the signal isdemodulated by demodulator 344 to produce the original I and Qcomponents. De-mapper 346 translates the I and Q components into encodedprimary and secondary data streams. These encoded bit streams, whichwere interleaved by interleaver 316, are recovered by de-interleaver 347and passed to decoder 348. Decoder 348 employs known bit and blockdecoding methods to decode the primary and secondary bit streams toproduce the original input signals containing the primary and secondarydata 302, 304. In other embodiments of the present invention, multipledecoders may be used, e.g., outer and inner decoders. Receiver 340 mayalso use a feed forward correction technique to improve its detection ofthe secondary data. By knowing the relative I/Q quadrant, receiver 340may be enhanced to perform better by having such a priori knowledge,which assists in the detection of the transmitted signal. Simulationshave shown that the use of first level data in the feed forwardcorrection technique enables the present invention to utilize correctedfirst level data symbols to optimize second level data performance aswell as potentially improve second level data performance over SDARcommunication systems utilizing non-feed forward correction techniques.For example, referring to FIG. 6, if it is known from a priori firstlevel data knowledge that symbol 602 or 601 was transmitted at somepoint in time, and the received symbol lands at 604, it can be inferredby minimum distance that the received second level data bit is a weakone (1) by utilizing feed forward correction. However, without feedforward correction the second level data bit would have been detected asa strong zero (0). Therefore, feed forward detection utilizes thedecoded symbol with the detected offset (either positive or negative) todetermine the secondary data bit.

In another embodiment of the present invention, a method of enablingextra data bits from a hierarchical modulation scheme to be used totransmit additional data for each channel in a SDAR system iscontemplated. A flow chart illustrating this embodiment of the presentinvention as utilized in an SDAR communication system is shown in FIG.7. It is contemplated that the inventive method would be carried out bya receiver adapted to be used in a SDAR system. The receiver mayconcurrently process the receipt of first data stream 710 and seconddata stream 730. If first data stream 710 is valid as determined byerror checking at step 712, first data stream 710 is passed to a channeldata select at step 750. If first data stream 710 is selected and seconddata stream 730 is either independent or not valid, only first datastream 710 is decoded at step 720 at its original rate, e.g.,forty-eight (48) kbps. The decoded data from first data stream 710 isthen passed to an output unit at step 724.

If second data stream 730 is valid as determined by error checking atstep 732, then second data stream 730 is passed to the channel dataselect at step 750. If second data stream 730 is selected and isindependent from first data stream 710, only second data stream 730 isdecoded at step 740 at its original rate, e.g., sixteen (16) kbps. Thedecoded data from second data stream 730 is then passed to an outputunit at step 744.

If the receiver determines at step 712 that first data stream 710 isvalid and at step 732 that second data stream 730 is valid, both datastreams are passed to the channel data select at step 750. The channeldata select determines if second data stream 730 is an enhancement tofirst data stream 710. Audio enhancements may include audio qualityenhancements, audio coding enhancements such as 5.1 audio (i.e., aDolby®) AC-3 digital audio coding technology in which 5.1 audio channels[left, center, right, left surround, right surround and alimited-bandwidth subwoofer channel] are encoded on a bit-rate reduceddata stream), data/text additions, album pictures, etc. If second datastream 730 is an enhancement to first data stream 710, the channel dataselect combines the two (2) data streams such that the combined signalhas a data rate greater than the first data stream's 710 data rate,e.g., 64 kbps. Thus, the sixteen (16) kbps data rate of second datastream 730 acts to increase the rate of first data stream 710 fromforty-eight (48) kbps to sixty-four (64) kbps. Combined data stream 758is then decoded at step 752 and passed to an output unit at step 756. Inan exemplary embodiment, when switching from first data stream 710 tocombined data stream 758, the increase in data rate is blended so as notto enable a quick change between first data stream 710 and combined datastream 758. If second data stream 730 is determined to be invalid, thechannel data select switches to a “first data level” only implementationand sends first data stream 710 to be decoded at step 720. The data rateof first data stream 710 remains at its original forty-eight (48) kbps.In an exemplary embodiment of this inventive method, a decrease in datarate is blended so as not to enable a quick change between first datastream 710 and combined data stream 758. Assuming that second datastream 730 becomes or remains valid, the receiver decodes combined datastream 758 at step 752 and provides combined data stream 758 to anoutput unit at step 756.

While this invention has been described as having an exemplary design,the present invention may be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains.

1. In a digital data transmission system including a transmitter fortransmitting transmit signals, a receiver comprising: a receiving unitincluding satellite signal detection means for detecting a firsttransmit signal transmitted from a first communication satellite and asecond transmit signal transmitted from a second communicationsatellite, the first transmit signal produced when the transmittermodulates a primary data stream with a secondary data stream on a firstcarrier wave associated with the first communication satellite and thesecond transmit signal produced when the transmitter modulates theprimary and secondary data streams on a second carrier wave associatedwith the second communication satellite, said receiving unit furtherincluding a terrestrial signal detection means for detecting a thirdtransmit signal transmitted from a terrestrial repeater, the thirdtransmit signal produced when the terrestrial repeater receives at leastone of the first and the second transmit signals and modulates theprimary data stream with the secondary data stream on a third carrierwave associated with the terrestrial repeater; and at least onedemodulator coupled to said receiving unit and configured to demodulateat least one of the first and the second transmit signals, wherein theterrestrial signal detection means include an antenna in communicationwith a tuner unit for receiving the third transmit signal, wherein thethird transmit signal is produced when the primary and the secondarydata are re-encoded on the third carrier wave using an OFDM modulationscheme, the OFDM modulation scheme dividing the third carrier wave intomultiple sub-carrier waves each having one of a M-PSK, D-MPSK and pi/4D-MPSK modulation scheme.
 2. In a digital data transmission systemincluding a transmitter for transmitting transmit signals, a receivercomprising: a receiving unit including satellite signal detection meansfor detecting a first transmit signal transmitted from a firstcommunication satellite and a second transmit signal transmitted from asecond communication satellite, the first transmit signal produced whenthe transmitter modulates a primary data stream with a secondary datastream on a first carrier wave associated with the first communicationsatellite and the second transmit signal produced when the transmittermodulates the primary and secondary data streams on a second carrierwave associated with the second communication satellite; and at leastone demodulator coupled to said receiving unit and configured todemodulate at least one of the first and the second transmit signals,wherein said receiving unit is operable to receive the first and thesecond transmit signals including secondary data comprising modulateddata in the form of a phase offset relative to the modulated primarydata.
 3. The receiver of claim 2 wherein said receiving unit is operableto receive the first and the second transmit signals including secondarydata comprising BPSK modulated data.
 4. The receiver of claim 2 whereinsaid receiving unit is operable to receive the first and the secondtransmit signals including primary data encoded with a first number ofinformation bits per symbol.
 5. The receiver of claim 4 wherein saidreceiving unit is operable to receive the first and the second transmitsignals including secondary data encoded with a second number ofinformation bits per symbol lower than the first number of informationbits.
 6. The receiver of claim 5 wherein said receiving unit is operableto receive the first and the second transmit signals including primarydata encoded with 2 bits per symbol.
 7. The receiver of claim 6 whereinsaid receiving unit is operable to receive the first and the secondtransmit signals including secondary data encoded with 1 bit per symbol.8. The receiver of claim 7 wherein said receiving unit is operable toreceive the third transmit signal encoded with a third number ofinformation bits per symbol, the third number of information bits persymbol greater than the first and the second numbers of bits per symbol.9. The receiver of claim 8 wherein said receiving unit is operable toreceive the third transmit signal encoded with 3 bits per symbol. 10.The receiver of claim 2 wherein said receiving unit is operable toreceive the first and the second transmit signals including secondarydata spread over multiple symbols.
 11. The receiver of claim 2 whereinsaid receiving unit is operable to receive the first and the secondtransmit signals including secondary data spread over a multiple of 2symbols.
 12. The receiver of claim 10 wherein said receiving unit isoperable to receive the first and the second transmit signals includingsecondary data encoded to ensure a zero mean offset over the multiplesymbols such that for each positive phase offset, there is acorresponding negative phase offset.
 13. A method of receivingtransmitted data, the method comprising the steps of: detecting a firsttransmit signal, the first transmit signal transmitted from a firstcommunication satellite and comprising a first level data and a secondlevel data modulated on a first carrier wave associated with the firstcommunication satellite; detecting a second transmit signal, the secondtransmit signal transmitted from a second communication satellite andcomprising the first level data and the second level data modulated on asecond carrier wave associated with the second communication satellite;detecting a third transmit signal, the third transmit signal transmittedfrom a terrestrial repeater and comprising the first level data and thesecond level data re-encoded on a third carrier wave associated with theterrestrial repeater; demodulating the first and the second transmitsignals; interpreting the first transmit signal; and providing theinterpreted signal to an output unit, wherein the first level datacomprises one of QPSK, D-QPSK and pi/4 D-QPSK modulated data andincludes a first number of information bits per symbol; and the secondlevel data comprises BPSK modulated data in the form of a phase offsetrelative to the first level data and includes a second number ofinformation bits that is lower than the first number of informationbits.